1. Field of the Invention
The present invention relates generally to a method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to a multifrequency method for reducing the effect of any ringing, such as magnetoacoustic ringing, and DC offset, during a nuclear magnetic resonance measurement.
2. Background of the Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A magnet on the NMR instrument is used to induce a static magnetic field in the earth formation. The static magnetic field aligns the nuclear spins of nuclei, particularly including hydrogen nuclei, in the formation in a direction parallel to that of the static field.
The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the field applied by the magnet. This RF pulse sometimes has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly both to the orthogonal magnetic field induced by the RF pulse and to the magnetic field applied by the magnet. After the pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually xe2x80x9crelaxxe2x80x9d or return to their original alignment with the magnet""s field. The amount of time taken for this relaxation, referred to as T1, is related to petrophysical properties of interest of the earth formation.
After the pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The precessional rotation generates RF energy at a frequency proportional to the strength of the magnetic field applied by the magnet, this frequency being referred to as the Larmor frequency. The constant of proportionality for the Larmor frequency is known as the gyromagnetic ratio xcex30. The gyromagnetic ratio is unique for each different chemical elemental isotope. The spin axes of the hydrogen nuclei gradually xe2x80x9cdephasexe2x80x9d because of inhomogeneities in the magnet""s field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID). The rate of FID is typically referred to by the notation T2*. The FID decay rate consists of a first component, referred to as xe2x80x9ctrue T2xe2x80x9d, which is due to internal nuclear environmental effects, and a second component resulting from microscopic differences in magnetic field and inhomogeneities in the earth formation. The effects of the second component can be substantially removed by a process referred to as spin-echo measurement.
One problem with analysis of NMR measurements is that the signal detected by the antenna includes a parasitic, spurious ringing that interferes with the measurement of spin-echoes. One source of the spurious signal is electromagnetic generation of ultrasonic standing waves in metal. The induced RF current within the skin depth of the metal interacts with the lattice in a static magnetic field through the Lorenz force and the coherent ultrasonic wave propagates into the metal to set up a standing wave. A reciprocal mechanism converts the acoustic energy, in the presence of the static field, to an oscillating magnetic field which is picked up by the antenna as a spurious, ringing signal.
Different types of magnetoacoustic interaction may produce a parasitic signal in the NMR antenna. Antenna wiring and other metal parts of the NMR logging tool can be affected by the static magnetic field and the RF field generated by the antenna. If the antenna is located within the strongest part of the magnet""s field, when RF pulses are applied to the antenna, acoustic waves are generated in the antenna and the antenna sustains a series of damped mechanical oscillations in a process known to those skilled in the art as magnetoacoustic ringing. This ringing can induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
Another source of magnetoacoustic interaction is magnetorestrictive ringing which is typically caused when nonconductive magnetic materials, such as magnetic ferrite, are used in the antenna. If this magnetic material is located within the strong part of the RF field, application of RF pulses will generate acoustic waves in the magnet. The magnet will experience a series of damped mechanical oscillations upon cessation of the RF pulse. Magnetorestrictive ringing can also induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
One approach to reduce the effects of ringing has been to design the hardware to minimize the interaction between the electromagnetic fields and the materials in the device. For example U.S. Pat. No. 5,712,566 issued to Taicher et al. discloses a device in which the permanent magnet composed of a hard, ferrite magnet material that is formed into an annular cylinder having a circular hole parallel to the longitudinal axis of the apparatus. One or more receiver coils are arranged about the exterior surface of the magnet. An RF transmitting coil is located in the magnet hole where the static magnetic field is zero. The transmitting coil windings are formed around a soft ferrite rod. Thus, magnetoacoustic coil ringing is reduced by the configuration of the transmitting coil. Magnetorestrictive ringing of the magnet is reduced because the radial dependence of the RF field strength is relatively small due to use of the longitudinal dipole antenna with the ferrite rod. Further, magnetorestrictive ringing is reduced because the receiver coil substantially removes coupling of the receiver coil with parasitic magnetic flux due to the inverse effect of magnetostriction.
Another commonly used approach to reduce the effect of ringing is to use a so-called phase-alternated-pulse sequence. Such a sequence is often implemented as
RFAxc2x1xxe2x88x92xcfx84nxc2x7(RFByxe2x88x92xcfx84xe2x88x92echoxe2x88x92xcfx84)xe2x88x92TWxe2x80x83xe2x80x83(1)
where RFAxc2x1x is an A pulse, usually 90xc2x0 tipping pulse and RFB is a refocusing B pulse. The xc2x1 phase of RFA is applied alternately in order to identify and eliminate systematic noises, such as ringing and DC offset through subsequent processing. By subtracting the echoes in the xe2x88x92 sequence from the pulses in the adjoining + sequence, the ringing due to the 180xc2x0 is suppressed.
The minimum acquisition time for a phase-alternated pair (PAP) is the sum of two CPMG sequence times and the wait time between the two sequences. For a CPMG sequence, the RFB pulse is a 180xc2x0 pulse. Typically, the wait time is about three times the maximum spin-lattice relaxation time (T1) of interest. The length of the CPMG sequence (AT) is typically of the order of the maximum transverse relaxation time (T2). Because there are significant static field gradients associated with NMR logging tools, the CPMG sequence is usually much shorter than the wait time. The minimum acquisition time TPAP for a PAP is approximately given by
TPAP=AT+TW xe2x89xa1TR≈AT+3T1|max≈3T1|maxxe2x80x83xe2x80x83(2)
In wireline applications, the tool moves at a vertical speed of VL. As a result of the tool motion, the minimum vertical resolution Rv of a moving tool is given by
Rv|min≈A+VLTPAPxe2x80x83xe2x80x83(3)
where A is the antenna aperture length. The best resolution can be obtained only when the fluids in the formation relax quickly or when the logging speed is small. In actual operation, a single PAP does not have sufficient signal-to-noise ratio (SNR) to be useful and more than one PAP is required: this further degrades the resolution.
Multi-volume tools have an advantage over single volume tools because they can fill the wait time between halves of a PAP with acquisitions for other sensitive volumes. The operation of multi-volume tools is best understood by reference to FIG. 1.
FIG. 1 shows a graph of the amplitude of the static magnetic field, with respect to distance from the magnet, for a well logging apparatus that has a gradient magnetic field. The amplitude of the static magnetic field generally decreases with respect to the lateral distance from the magnet. As is well known in the art, nuclear magnetic resonance conditions occur when a radio frequency magnetic field is applied to materials polarized along a static magnetic field where the frequency of the RF magnetic field matches the product of the static magnetic field strength and the gyromagnetic ratio of the nuclei being polarized by the static magnetic field, this product being referred to as the Larmor frequency. As can be inferred from the graph in FIG. 1, by adjusting the frequency of the RF magnetic field, the distance from the magnet at which nuclear magnetic resonance conditions occur can be changed corresponding to the static magnetic field amplitude at that particular distance from the magnet. For example, if frequency f1 is the highest frequency, resonance will occur at the smallest distance to the magnet, and so on through lower frequencies f2 through fN. Because nuclear magnetic resonance only occurs where the static magnetic field strength matches the RF magnetic field frequency, nuclear magnetic resonance measurements can be conducted within a number of different non-overlapping sensitive volumes by inducing nuclear magnetic resonance at different frequencies. An example of a gradient tool is described in U.S. Pat. No. 5,712,566 to Taicher et al. The Taicher ""566 device gives non-overlapping sensitive volumes comprising thin annular cylinders each having an average radius corresponding to the particular static magnetic field amplitude in which nuclear magnetic resonance would occur at a particular RF magnetic field frequency. The thickness of each annular cylinder would be related to the bandwidth of a receiver circuit in the NMR instrument and the rate at which the static magnetic field changes in amplitude.
Examples of multi-volume PAPs are given, for example, in U.S. Pat. No. 6,049,205 to Taicher et al, the contents of which are fully incorporated herein by reference. While multi-volume PAPs measurements are more efficient at power utilization, the resolution is still controlled by TPAP. This is best understood by reference to FIG. 2. Shown in FIG. 2 is an example of the acquisition of six PAP sequences for six different volumes and six corresponding frequencies. The abscissa is the time in milliseconds. The acquisition sequence is indicated with the six different volumes separated. As an example, pulse sequence denoted by 101a acquires data at from a volume V1 at a frequency f1. This is then followed by the pulse sequences denoted by 103a for a volume V2 at a frequency f2, 105a for a volume V2 at a frequency f2, through 111a for a volume V6 at a frequency f6. This comprises a first half of a PAP sequence for six volumes. The phase of the RF signal is then reversed and the sequence of pulses 101b. 103b, 105b, 107b, 109b and 111b are acquired for the same sequence of six volumes V1, V2 . . . V6 at frequencies f1, f2 . . . f6. The wait time Tw between the two halves of a PAP sequence for a particular volume is 4.5 seconds and is the same for all volumes. In the example shown, the acquisition time AT for each CPMG sequence is the same (0.8 seconds). Those versed in the art would recognize that each individual component of a PAP pair must occur at the same frequency because the ringing characteristics depend upon the frequency; attempting to combine CPMG sequences at different frequencies will results in incomplete subtraction of the ringing signal.
The resolution of the measurement can be calculated from number of CPMG sequences NA needed to reach a SNR threshold, the number of volumes Nv, and TPAP. It is given by                               R          v                =                  A          +                                                    V                L                            ⁡                              (                                  1                  +                                                                                    NA                        /                        2                                            -                      1                                                              N                      v                                                                      )                                      ⁢                                          T                PAP                            .                                                          (        4        )            
The need for PAPs requires that NA be a multiple of two. The resolution is proportional to the inverse of the number of volumes. The minimum resolution from eq. (4) is for NA=2 and is the same as that given by eq. (3) regardless of the number of volumes and frequencies. Thus, using multiple volumes with corresponding frequencies may improve the power utilization, but it does not improve the resolution of an NMR logging tool.
It would be desirable to have a method of NMR data acquisition that is able to suppress ringing while improving the power efficiency of a single frequency CPMG sequence. The present invention satisfies this need.
The present invention is a method for determining a parameter of interest of an earth formation with a gradient Nuclear Magnetic Resonance (NMR)tool conveyed in a borehole. A static magnetic field is produced within a first region in the formation, preferably using a permanent magnet. NMR spin-echo signals from the first region by using a transmitter on the tool with a first pulsed radio frequency (RF) signal having a Larmor frequency corresponding to the field strength in the first region as the first half of a phase alternated pair (PAP) of measurements. An electromagnet on the tool is used to alter the static field to alter the static field so that the altered static field has the same field strength (and Larmor frequency) in a second region of the formation that is non-overlapping with the first region and the second half of the PAP is obtained. The combination of the two echo sequences can reduce the effects of ringing. This may be repeated for additional regions with different Larmor frequencies. Due to the non-overlapping of the two regions, depending upon the number of repetitions needed to obtain adequate signal to noise ratio, the total acquisition time using field and frequency shifting may be significantly less that for prior art methods that only use frequency shifting.